The invention relates to a multiple-shaft mixer/reactor with a large free usable volume which cleans itself kinematically, consisting of two or more parallel shafts rotating in opposite directions, on which are mounted, helically offset, feed blades which are connected to one other by means of axially extended kneading bars, and a surrounding casing as well as optionally an inlet and an outlet for the material to be mixed.
The invention is geared towards devices for the treatment by process technology of fluids and cohesive bulk goods. The device is fully self-cleaning kinematically and has a large free usable volume.
In, inter alia, the manufacture and processing of synthetic materials, highly viscous liquids have to be treated by process technology. In particular, appliances are required for the purpose of mixing and evaporating. These need to bring about a good mixing action and also, in the case of evaporation, a rapid renewal of the free surfaces of the mixer.
Deposits of product on the walls of such mixers can result in the process being impaired. Undesirable side reactions in the deposits are favoured by virtue of the considerably prolonged dwell-time in the reactor. This results in contamination of the product. Product deposits on the walls can be avoided through kinematic self-cleaning of the mixer.
By way of example, mention may be made of the production of thermotropic liquid-crystal polyesters. A factor determining the speed in the final stage of polycondensation is the mass transfer of the condensate into the gas phase of the reactor, which is subject to vacuum. This requires a mass-transfer surface which is as large as possible and a renewal of the same which is as rapid as possible. By reason of the pronounced intrinsic viscosity the product has a tendency to form wall deposits in non-agitated zones. As a result of the longer dwell-times prevailing here, black cracking products arise which result, if they get into the product flow, in goods that cannot be marketed.
With a view to minimising the production and operating costs of a reactor/mixer, a large free usable volume is additionally striven for--i.e., the ratio of the volume of the stirrer unit to the volume of the casing should be as low as possible.
These requirements are to a certain extent satisfied by the apparatus described in published application DE 41 26 425 A1.
However, the apparatus described therein has two serious defects:
Only the shaft of the rotors serves to absorb bending forces, for instance in the course of mixing pasty fluids. The shaft, however, should be as thin as possible in order to obtain a large free usable volume. As a result, given the small clearances which are also striven for (on account of the self-cleaning), by reason of the deflection of the shaft it is barely possible to attain a ratio of apparatus length to shaft distance of more than 5, at best 7.
Heating of the blades and gear wheels would only be possible in the stated mixing device by means of a complicated guiding of the heating channels, whereby flow and return lines have to be guided through each tooth base. From the point of view of process technology this can be controlled only with difficulty or is associated with high costs.